Dial Vs Digital Indicators

In the 1980s, digital electric indicators were expected to blow mechanical dial indicators out of the water. Despite electronic indicators’ higher resolutions, better accuracy and usefulness in statistical process control and data collection systems, mechanical indicators retained other advantages and continued to be specified by many users.

Columns Post: 2/15/2011

George Schuetz

Director of Precision Gages, Mahr Inc.

Indicator choice depends on the application and user preference. Today, while neither type is “better” than the other, digital finally seems to be gaining the upper hand.

Digital indicators have a clear advantage for data collection in process control applications. They can output measurements directly, usually as an actual value, such as 20.00 mm, to printers or SPC programs with no operator errors in reading or recording. The operator simply positions the workpiece and presses a button. The digital process has even gone wireless within the past five years.

With dial indicators, the operator must interpret the pointer’s position to read the measurement, record it and key the data into a computer. That is three steps during which errors can occur. If data must be entered into a computer system, digital indicators are the best resource.

However, even without the cost benefit, there is something to be said for mechanical dial indicators. I have seen QC inspectors make consistently accurate go/no-go readings with dial indicators before the pointer has stopped moving. At a glance, they can tell approximately where the pointer will stop. This accuracy is close enough for many applications. Digital displays don’t give users the option of approximating. When a digital device is flickering between six and seven, all of the elements in an LCD display may be lit, appearing as an eight.

Skilled operators can also “split grads” with dial indicators. This means they can resolve the pointer’s position to an accuracy of about one-fifth of the gage’s stated minimum graduation value. Analog dials enable the machinist to observe the direction the process is headed. If reading No. 1 indicates one-fifth of a grad over zero, reading No. 2 is precisely zero, and reading No. 3 is one-fifth of a grad below zero, then the user might be able to draw valuable conclusions about the condition of his tool. In other words, dials can provide more information than just the dimensional measurement. An early digital readout would have read zero in all three cases, depriving the user of this additional information.

On the other hand, today’s digital indicators are capable of resolutions that were once reserved for bench amplifiers. Now that “fifth of a grad” is another digit on the indicator, and that process change can be observed (assuming that the gage is up to the higher resolution of the display). In addition, many of today’s digital indicators have some form of supplemental analog display. These electronic emulations of analog performance eliminate some of the cognitive disadvantages of digital displays. They also make digital indicators “user-friendly” because they give the impression of direction and how far the inspected item is over or under tolerance.

In fact, many digital indicators are becoming so powerful they are taking on bench amplifier performance. It is not uncommon to find such features as dynamic measurements, multiple factors, unilateral tolerances, different output formats, and microinch resolutions in high-end digital indicators. While indicators with these features are considered high-end, they are still about a quarter of the price of a bench amplifier and probe.

A common serious problem among dial indicator users is the failure to notice when the pointer makes a full revolution or two. Parts that are grossly out of tolerance may appear to be within tolerance to an inattentive operator. In contrast, digital indicators never come “back to zero,” eliminating this problem entirely. In fact, with the ability to display the actual part size, the concept of going back to “zero” or reading deviations is fading. Plus, all digital indicators can be made to signal out-of-tolerance dimensions, so a user is rarely required to make decisions.

Despite initial doubts, digital indicators have proven to be highly reliable in the shopfloor environment. Most have only a single moving part, so they require less frequent cleaning than their mechanical cousins. Many now carry IP ratings that define whether they can be used in dusty, wet or other adverse environments. However, dial indicators can last virtually forever with proper care, and they never need batteries. A drawback is that it is increasingly difficult to find people who can repair dial indicators.

Back in the ‘80s, the marketplace soundly rejected digital speedometers in cars. Today, as more and more people are being raised in a digital world, there is a resurgence of digital speedometers, tachometers and other indicators. The same is true for digital indicators. As operators grow more used to these devices, they are employed in more and more applications.